import pygame
from pygame.sprite import Sprite


class Bullet(Sprite):
    
    def __init__(self, ai_game):
        """ 创建一个子弹对象 """
        super().__init__()
        self.screen = ai_game.screen
        self.settings = ai_game.settings
        self.color = self.settings.bullet_color
        """ 将子弹置于飞船上方发射 """
        self.rect = pygame.Rect(0, 0, self.settings.bullet_width,
                                self.settings.bullet_height)
        self.rect.midtop = ai_game.ship.rect.midtop

        """ 存储子弹位置 """
        self.y = float(self.rect.y)

    def update(self):
        """ 向上移动子弹,向上发射时y坐标依次减 """
        self.y -= self.settings.bullet_speed
        self.rect.y = self.y

    def draw_bullet(self):
        """ 在屏幕上绘制子弹 """
        pygame.draw.rect(self.screen, self.color, self.rect)